Process for the preparation of a pigment suspension

ABSTRACT

Pigment suspensions of inorganic pigments are prepare compacting and grinding the unground pigments on a roller compactor, followed by processing of the compacted pigment in a wet grinding mill with addition of water. The process as described improves the particle size distribution of the pigment.

The present invention relates to a process for the preparation of apigment suspension from of one or more inorganic pigments and water.

Ground pigment powders and water are generally used for preparinginorganic pigment suspensions. Small quantities of organic or inorganicdispersing agents must sometimes be added.

Pinned disc mills, impeller ring mills, pendulum mills or jet mills arecustomarily used for grinding the unfinished pigments for thepreparation of TiO₂ or iron oxide pigments (Ullmanns Encyclopadie dertechnischen Chemie, 4th new, revised and enlarged Edition, Volume 18,pages 576 and 601).

The ground pigment powders are then dispersed in stirrer vessels, insome cases with the aid of dispersing agents. These vessels may befitted with intensive mixing devices. The finished suspension may thenbe marketed as trade product or subjected to further processing for morecomplete extraction, depending on the purpose for which the pigment isrequired (Winnacker, Kuchler--Chemische Technologie, Volume 3,Anorganische Technologie II, 4th Edition, pages 371 and 402).

A disadvantage of the usual procedure is that the ground pigments can insome cases only be dispersed with a great expenditure of energy as theytend to form lumps when they undergo dispersion. Additional measuressuch as screening are required for removing agglomerates or aggregateswhich have not been ground down. Further disadvantages lie in the factthat the pigment must be used as a powder, which produces such a greatdeal of dust when incorporated in the end product that additionalmeasures must be carried out to prevent excessive dust formation.

It is therefore an object of the present invention to provide a processfor the preparation of pigment suspensions which is free from theabove-mentioned disadvantages.

It is surprisingly found that this can be achieved by first compactingthe unground crude pigment on a roller compacter by which it is groundat the same time, and then treating the resulting compacted granulate,which is relatively free from dust, with water in a wet grinding mill toform the pigment suspension.

This invention thus relates to a process for the preparation of apigment suspension from one or more inorganic pigments and water,characterised in that the unground pigment is first compacted and groundon a roller compacter and the compacted pigment powder is then treatedin a wet grinding mill with the addition of water to produce thefinished pigment suspension.

Compacting is preferably carried out at linear forces of from 1 to 60KN/cm. The wet grinding is preferably carried out in vibrating ballmills (intermittently or continuously) or pearl mills (upright orhorizontal).

Exceptionally good results are obtained when the solids content of thesuspension amounts to 5 to 75% by weight, preferably from 15 to 65% byweight.

It is particularly advantageous to carry out the process according tothe invention in the presence of from 0.05 to 5% by weight of organic orinorganic additives as dispersing agents, preferably phosphates, basedon the solids content.

It may also be advantageous to thicken the finished pigment suspensionto a higher solids content than the starting suspension by filtrationand/or sedimentation.

The pigment suspension obtained by the process according to theinvention contains a very small proportion of particles greater than 40μm and has an improved particle size distribution. The particle sizedistribution is determined according to Ullmanns Encyclopedia ofIndustrial Chemistry, Fifth, Completely Revised Edition, Volume B 2,Unit Operations I, Kurt Leschonski, 2. Particle Size Analysis andCharacterisation of a Classification Process (pages 2-30).

To determine the sieve residues, 1 liter of the suspension wasintroduced into a 40 μm sieve which was then rinsed with 10-15 ml ofwater. The sieve was then dried and weighed. The percentages by weightrefer to the weight differences of the sieve.

The process according to the invention is applicable to all inorganicpigments and fillers. Iron oxides, titanium dioxide, chromium oxideand/or mixed phase pigments are particularly preferred inorganicpigments.

The invention is described below with the aid of Examples, which are notto be regarded as limiting.

EXAMPLE 1

A titanium dioxide raw material (starting material for the Trade Productof Bayer AG, Bayertitan® R-KB-2), i.e. a product withoutafter-treatment, was compacted on a commercial roller mill with a linearforce of 40 kN/cm without the addition of a grinding auxiliary.

The dust-free compacted granulate obtained by compacting was thendisagglomerated for one hour in the form of a suspension containing 20%by weight of TiO₂ in a vibrating ball mill containing a 50:50 mixture of8 mm and 12 mm steatite balls. A thin suspension leaving a residue of0.9% by weight on a 40 μm sieve was obtained.

When a pigment suspension containing 20% by weight of TiO₂ was preparedby conventional grinding in a stirrer vessel, the residue on the 40 μmsieve was 1.4% by weight.

A comparison between the particle size distributions shows theimprovement in the reduced proportion of coarse particles (C) and thereduction in the range of particle sizes (C-A/B).

Data in which the particle size in μm is smaller by 10%, 50% or 90% ofthe total quantity than the given numerical value.

    ______________________________________                                                                        Range of particle                                       10%   50%     90%     size distribution                                       A     B       C       C-A/B                                         ______________________________________                                        earlier process                                                                           0.19    0.41    0.91  1.78                                        process according                                                                         0.22    0.39    0.68  1.17                                        to the invention                                                              ______________________________________                                    

EXAMPLE 2

Compacting of the crude pigment TiO₂ was carried out as in Example 1 butwith a linear force of 25 kN/cm. A suspension containing 25% by weightof TiO₂ and 0.2% by weight of sodium hexametaphosphate was then preparedand disagglomerated in a continuously operating vibrating ball mill(Palla mill) containing 2.5-3.5 steatite balls. The residence time ofthe suspension in the mill was 15 minutes. The sieve residue on the 40μm sieve was 0.05% by weight.

The residue obtained on a 40 μm sieve from a pigment suspensioncontaining 25% by weight of TiO₂ prepared by conventional grinding in astirrer vessel was 0.9% by weight. A comparison of the particle sizedistributions shows the improvement obtained, i.e. the reducedproportion of coarse particles (C) and the reduction in the range ofparticle sizes (C-A/B).

Data in which the particle size in μm is smaller by 10%, 50% or 90% ofthe total quantity than the given numerical value:

    ______________________________________                                                  10%   50%     90%     Range of particle                                       A     B       C       sizes C-A/B                                   ______________________________________                                        earlier process                                                                           0.19    0.42    0.93  1.76                                        process according                                                                         0.25    0.38    0.59  0.91                                        to the invention                                                              ______________________________________                                    

EXAMPLE 3

An unground red iron oxide pigment (starting material for the TradeProduct of Bayer AG, Bayferrox® 130) was compacted with a linear forceof 30 kN/cm. The dust-free granulate obtained by compacting was thendisagglomerated in the form of a suspension with a 50% by weight solidscontent in a vibrating ball mill as in Example 1. A pigment suspensionleaving a residue of 0.3% by weight on the 40 μm sieve was obtained.

A pigment suspension containing 50% by weight of red iron oxide preparedby conventional grinding in a stirrer vessel leaves a residue of 0.8% byweight on the 40 μm sieve. Comparison of the particle size distributionsshows the improvement in the reduced proportion of coarse particles (C)and the reduction in the range of particle sizes (C-A/B).

Data in which the particle size in μm is smaller by 10%, 50% or 90% ofthe total quantity than the given numerical value:

    ______________________________________                                                  10%   50%     90%     Range of particle                                       A     B       C       sizes C-A/B                                   ______________________________________                                        earlier process                                                                           0.29    0.48    0.73  1.24                                        process according                                                                         0.31    0.45    0.68  0.81                                        to the invention                                                              ______________________________________                                    

What is claimed is:
 1. A process for the preparation of a pigmentsuspension from one or more inorganic pigments and water, comprisingfirst compacting and grinding unground pigment on a roller compactor,followed by processing the compacted pigment powder in a wet grindingmill with the addition of water to produce the finished pigmentsuspension.
 2. A process according the claim 1, wherein compacting iscarried out at linear forces of from 1 to 60 kN/cm.
 3. A processaccording to claim 1, wherein wet grinding is carried out in vibratingball mills or pearl mills.
 4. A process according to claim 1, whereinthe solids content of the suspension is from 5 to 75% by weight.
 5. Aprocess according to claim 4, wherein the solids content of thesuspension is from 15 to 65% by weight.
 6. A process according to claim1, wherein the process is carried out in the presence of from 0.05 to 5%by weight of organic or inorganic additives as dispersing auxiliaries,based on the solids content.
 7. A process according to claim 6, whereinthe additive is a phosphate.
 8. A process according to claim 1, furthercomprising thickening the finished pigment suspension to a higher solidscontent by filtration or sedimentation.
 9. A process according to claim1, wherein the inorganic pigments are iron oxides, titanium dioxide,chromium oxide, mixed phase pigments, or mixtures thereof.